1. Field of the Invention
The present invention relates to a communication system for enabling communication through a human body or the like and a receiver used in the communication system.
2. Description of the Related Art
A communication device which communicates through tissue of a living body such as a human body is known. For example, a technique is known in which data can be exchanged by a user merely holding a hand over a receiver while a portable electronic device such as a portable phone on which a transmitter is mounted is placed in a pocket of clothing of the user, or while the portable electronic device is hung from the neck.
For example, as shown in FIGS. 10A and 10B, a transmitter 100 comprises an encoder 10, a transmission amplifier 12, an environment-side electrode 14, and a living body-side electrode 16, and a receiver 102 comprises a decoder 18, a reception amplifier 20, an environment-side electrode 22, and a living body-side electrode 24. The transmitter 100 is mounted on a portable electronic device or the like which is carried by the user. The receiver 102 is placed on a ticket barrier of a station, a vending machine, a shop, etc.
FIG. 11 shows a relationship between the transmitter 100, the receiver 102, and the human body or the like during the communication. FIG. 12 shows an equivalent circuit of the relationship.
The transmitter 100 capacitively couples with the receiver 102 through tissue of a living body such as human body or the like (hereinafter simply referred to as “human body or the like”). The environment-side electrode 14 of the transmitter 100 forms a capacitive coupling A with an external environmental ground potential, a capacitive coupling B with the human body or the like, and a capacitive coupling D with an external environment. Similarly, the environment-side electrode 22 of the receiver 102 forms a capacitive coupling H with the external environmental ground potential and a capacitive coupling G with the external environment. As described, the environment-side electrodes 14 and 22 are electrodes which form capacitive couplings with the external environment during the communication.
The living body-side electrode 16 of the transmitter 100 forms a capacitive coupling C with the human body or the like. The living body-side electrode 24 of the receiver 102 forms a capacitive coupling F with the human body or the like. Moreover, a capacitive coupling E is formed between the human body or the like and the external environment. As described, the living body-side electrodes 16 and 24 are electrodes which form capacitive couplings with the human body or the like during the communication.
The transmission amplifier 12 of the transmitter 100 receives information encoded by the encoder 10 and outputs as a potential difference between the environment-side electrode 14 and the living body-side electrode 16. When the transmitter 100 and the receiver 102 are electrically coupled through the human body or the like as described above, the potential difference between the environment-side electrode 14 and the living body-side electrode 16 of the transmitter 100 causes a change in a potential difference between the environment-side electrode 22 and the living-body side electrode 24 of the receiver 102. The reception amplifier 20 of the receiver 102 amplifies the potential difference between the environment-side electrode 22 and the living body-side electrode 24 and outputs the amplified signal. The output of the reception amplifier 20 is decoded by the decoder 18. In this manner, the communication is established.
For example, communication is enabled by a user who carries the transmitter 100 holding a hand over (or contacting with a hand) the living body-side electrode 24 of the receiver 102 placed on a ticket barrier of a station.
In a portable terminal such as a portable phone and a PDA in the related art, as shown by a cross sectional diagram of FIG. 13, the environment-side electrode 14 and the living body-side electrode 16 are placed attached to an internal surface of a housing of the portable terminal and a circuit board 26 on which the reception amplifier 20, the decoder 18, etc. which process the signals from the environment-side electrode 14 and the living body-side electrode 16 are mounted is placed in the housing. In this process, in order to prevent influences of electromagnetic waves emitted from the circuit board 26 on the signals received on the environment-side electrode 14 and the living body-side electrode 16, a structure is employed in which the circuit board 26 is stored in a shield case 28 made of a conductor.
When a structure of storing the circuit board 26 in the shield case 28 is employed, the manufacturing cost is increased by the shield case 28, and in addition, as shown in an equivalent circuit of FIG. 14, the intensity of the signal which can be detected from the environment-side electrode 14 and the living body-side electrode 16 is reduced due to influences of parasitic capacitances C1 and C2 between the environment-side and living body-side electrodes 14 and 16 and the shield case 28.
In consideration of this, another configuration is employed in which, as shown in a cross sectional diagram of FIG. 15, the circuit board 26 is not stored in the shield case 28, but is placed between the environment-side electrode 14 and the living body-side electrode 16.
For example, when a reception signal as shown in FIG. 16A is obtained in the structure of FIG. 13, in a structure having a similar system but with the shield case 28 removed as shown in FIG. 15, the reception signal of FIG. 16B is obtained.
Thus, the shield case 28 becomes unnecessary and the parasitic capacitance between the environment-side and living body-side electrodes 14 and 16 and the circuit board 26 becomes smaller compared to the case where the shield case 28 is provided, so that the absolute intensity of the reception signal can be increased, but the electromagnetic waves transmitted from the circuit board 26 are superposed as noise on the signal received at the environment-side electrode 14 and the living body-side electrode 16, and the S/N ratio is degraded.
In particular, as the size of the portable terminal is reduced and the distance between the environment-side and living body-side electrodes 14 and 16 and the circuit board 26 is reduced, the influence of the electromagnetic noise transmitted from the circuit board 26 becomes more significant, and the problem of reduction in the S/N of the reception signal becomes more significant.